The Dominican Republic part is just fine. But you can get that same quality on the DR with Fodor's or Moon. However for the same price this book covers also Haiti.
As this is the only guidebook to Haiti there was a danger that it would be done very sloppy - with no competition. However the research is very thorough. Despite what some media want you to believe Haiti is safe for tourists (and also visited by many NGOs). To tell the truth official data shows that Haiti is safer then touristy Jamaica! It's also the best destination to see the old Caribbean as well as for history buffs (the Gingerbread houses, the Citadel).
This book covers Port-au-Prince, the urban spawl around it, all of southern Haiti, and most of the North (Goniaves, Cap-Haitien, Fort-Liberte, Tortuga). It doesn't cover some out of the way villages as well as Port-de-Paix, but still, it's very detailed for a Haiti book. Of course a full post earthquake update was done.
There is only ONE(!!!) alternative if you go to Haiti and it's not in English - it's the French "Petit Fute" guidebook, last published in... 2005. It is a little more detailed, but not worth it even if you speak French - it's just plain old at 6 years. The only thing it has slightly better then this book is the maps.